Digital combiners are presently proposed for digital RF communications networks. The digital RF networks currently being implemented fall into two general categories: time division multiple access (TDMA) and code division multiple access (CDMA). With respect to CDMA systems, a plurality of communication signals will be transmitted simultaneously over a particular bandwidth. In order to decorrelate the signals, each call is "spread" using a different spreading code, also known as a Walsh code. These spread signals are then combined and transmitted simultaneously.
A problem faced by present systems is that of the data rate which must be carried through the backplane of a transceiver. Presently, the signals to be transmitted are processed on a channel card which raises the data rate from 19.2 kilobits per second (kbps) to 11.1 Megabits per second (Mbps). Since there are 9 bits per sample, this data rate corresponds to a sample rate of 1.23 MHz per channel. Each of these channels is then transmitted from the channel card to a baseband digital combiner through a backplane. With 64 signals being transmitted, the data rate through the backplane is 708 Mbps. These 64 signals are then combined at the 1.23 MHz sample rate, and the combined signal sample rate is raised to 4.91 MHz through an up-sampling filter.
Therefore, a need exists for a transmitter design that will reduce the complexity of the channel card and reduce the data rate being conducted over a backplane.